The invention relates to a bicycle fork of the kind comprising a fork crown from where there extend two fork stems and, on the opposite side to the fork stems, a tubular pivot comprising a cylindrical bottom part secured to the fork crown, and a cylindrical top part for securing a handlebar stem.
It is known that, in a bicycle, the fork pivot is mounted to rotate in a bushing of the bicycle frame, generally by means of two rolling bearings provided one at the bottom and one at the top of the bushing. The cyclist, by acting on a handlebar fixed to the end of the handlebar stem, can thus steer the fork, and the front wheel, in a desired direction.
The connecting region linking the pivot and the fork crown constitutes a trouble spot and is particularly heavily mechanically loaded.
It is an object of the invention, above all, to provide a bicycle fork which, in the connecting region linking the fork crown and the pivot, has satisfactory rigidity and provides the best possible distribution of stress over the bearings, particularly at the bottom part of the pivot, without this resulting in an appreciable gain in the weight of the fork and of the steering gear.
According to the invention, a bicycle fork of the kind defined previously is characterized in that the outside diameter of the cylindrical bottom part of the pivot is greater than the outside diameter of the cylindrical top part of this pivot, and in that a transition region provides the link between the two, different-diameter, parts of the pivot.
This solution makes it possible to strengthen the connecting region linking the fork crown and the pivot, without in any way leading to an appreciable increase in weight because the top part of the pivot maintains a small outside diameter which involves no appreciable modification to the weight of the pivot and handlebar stem assembly.
As a preference, the transition region of the pivot has a frustoconical exterior surface, the outside diameter of which decreases from the bottom part toward the top part.
The fork is advantageously made of composite, preferably as a single piece with the pivot which is also made of composite.
The term xe2x80x9ccompositexe2x80x9d is intended to mean a material consisting of fibers of high mechanical strength, particularly carbon or glass fibers, embedded in a resin matrix. Parts manufactured from such a material are generally molded.
As an alternative, the pivot may be made of metal, particularly of light alloy. The fork may also be made of metal.
The pivot may be manufactured separately, then assembled with the fork crown.
As a preference, the diameter of the top part of the pivot is equal to a standard bicycle pivot diameter, particularly 25.4 mm or 28.6 mm. The diameter of the bottom part of the pivot is at least 5% (five percent), and advantageously at least 12% twelve percent), greater than the diameter of the top part.
The pivot may have a cylindrical cavity of circular cross section, the diameter of which remains constant from the bottom to the top.
The exterior surface of the connecting region linking the pivot and the fork crown advantageously has a frustoconical surface to act as a support for a mating surface of an inner ring of a rolling bearing. In the case of a fork and of a pivot made of composite, the fibers of the composite are continuous in the connecting region.
The invention also relates to a bicycle fork pivot comprising a cylindrical bottom part for linking to a fork crown and a cylindrical top part for the attachment of a handlebar stem, characterized in that the outside diameter of the cylindrical bottom part of the pivot is greater than the outside diameter of the cylindrical top part of the pivot, and in that a transition region provides the link between the two, different-diameter, parts.
Advantageously, the transition region of the pivot has a frustoconical exterior surface, the diameter of which decreases from the bottom upward.
The invention also relates to a bicycle steering gear, particularly integrated steering gear, comprising a fork as defined previously.
When the bicycle steering gear comprises a fork with a connecting region linking the pivot and the fork crown which has a frustoconical exterior surface, the steering gear comprises, at the bottom part of the frame bushing, a rolling bearing, the inner ring of which has an oblique bearing surface mating with the frustoconical surface of the linking region.